Compander system

ABSTRACT

A signal conditioning system comprises an improved gain control module 10 and an improved detector 12 which are adapted to be manufactured in integrated circuit form.

RELATED APPLICATION

This application is a continuation of our copending application Ser. No. 247,830 filed Mar. 26, 1981, abandoned.

This invention relates to signal companders and more particularly to an improved signal compander adapted to be formed as an integrated circuit (IC).

As well known compander systems are designed to compress the dynamic range of an audio signal prior to transmission over or recordation on a dynamic range limited medium; and to expand the dynamic range of the transmitted or recorded signal on subsequent playback so that the signal has greater dynamic range than otherwise provided by the transmitting or recording medium.

One such compander system which has been widely received as well as commercially successful is described in U.S. Pat. No. 3,789,143 issued Jan. 29, 1974 to David E. Blackmer and entitled Compander with Control Signal Logarithmically Related to the Instantaneous RMS Value of the Input Signal. Generally, this system utilizes two basic components, the gain control module or unit including a voltage control amplifier (VCA) and the signal detector used to generate the voltage control signal for the amplifier. The voltage control amplifier is preferably of the type described in U.S. Pat. No. 3,714,462 issued Jan. 30, 1973 to David E. Blackmer and entitled Multipler Circuits. The detector preferably includes circuit means for detecting the instantaneous RMS level of the input signal, such as described in U.S. Pat. No. 3,681,618 issued to David E. Blackmer on Aug. 1, 1972 and entitled RMS Circuits with Bipolar Logarithmic Converter.

Generally, the voltage control amplifier impresses a controlled gain on the input signal so as to either compress the dynamic range of the signal transmitted or recorded, or expand the dynamic range of the signal upon playback. The amount of compression or expansion is dependent upon the preselected compression or expansion factor (how much a signal is compressed or expanded). In the compander system of U.S. Pat. No. 3,789,143 the control signal is derived from a logarithmic function of the instantaneous RMS level of the input signal so as to provide expansion or compression as a function of the instantaneous RMS level of the input signal. It should be appreciated that other detection methods such as peak detection and averaging detection have been used.

It is a general object of the present invention to provide an improved compander system which is easy and inexpensive to manufacture.

Another object of the present invention is to provide a compander system having related improvements so as to be easily manufacturable in IC form.

And another object of the present invention is to provide a compander system having improvements over the one described in U.S. Pat. No. 3,789,143.

These and other objects are achieved by an improved compander system comprising an improved gain control module and an improved signal detector for providing a gain control signal to the gain control module in order to compress or expand the information signal depending upon the amplification setting of the unit.

Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

FIG. 1 is a block diagram of the most basic components of a compander system;

FIG. 2 is a block diagram of the essential components of the preferred embodiment of the compander system of the present invention;

FIGS. 3A and 3B are a circuit schematic of the preferred embodiment of the gain control module of the present invention; and

FIGS. 4A and 4B are a circuit schematic of the preferred embodiment of the detector of the present invention.

In the drawings the same numerals are used to refer to like parts.

In FIG. 1 the basic compander system such as the one described in U.S. Pat. No. 3,789,143 includes gain control module 10 and detector 12. Generally the gain control module adjusts the gain impressed on the input signal Ein applied to voltage input terminal 14 as a function of the control signal provided by detector 12. Depending on the gain setting of module 10, the gain impressed will provide either gain or attenuation of the input signal to provide the output signal Eout at output terminal 16. For compression the detector 12 can be connected in a feedback configuration (wherein the detector detects the output signal Eout at output terminal 16) or a feed forward configuration (wherein detector 12 detects the input signal Ein at voltage input terminal 14) both configurations being shown in FIG. 1. For expansion, the feed forward configuration is typically used. In both configurations, the detector provides the control signal in response to the detected signal.

Referring to FIG. 2, the preferred gain control module 10 includes input stage 18 having its input connected to voltage input terminal 14 and its output connected to VCA cell 20. The output of cell 20 is connected to output terminal 16 of the system and is suitably biased by bias generator 22.

Detector 12 shown connected with module 10 in the feedforward configuration comprises rectifier 24 for detecting the input signal at voltage input terminal 14 and for rectifying the input signal. Rectifier 24 has its output connected to logging section 26 for providing an output signal as a function of the logarithm of the rectifier output of rectifier 24. Logging filter 28 converts the log signal output of section 26 to a substantially DC signal which in turn is applied to the input of buffer amplifier 30. Amplifier 30 has its output connected to the control signal input of VCA cell 20.

Referring to FIGS. 3A and 3B, the gain control module is shown as including the input stage 18 (FIG. 3A), VCA cell 20 (FIG. 3B), bias generator 22 (FIG. 3B) and power supply 32 (FIG. 3A).

Referring specifically to FIG. 3A the current input terminal 14A is identical to voltage input terminal 14 of FIGS. 1 and 2 except that it is modified to receive the input current Iin as a function of the input voltage Ein in a manner well known in the art. Input terminal 14A is connected to the input of input stage 18 and the input of VCA cell 20. Input stage 18 is described in greater detail in our copending application Ser. No. 247,649 filed simultaneously herewith and assigned to the present assignee. Specifically, input terminal 14A is connected to the input terminal of stage 18 defined by junction 40. Junction 40 is connected to the base of buffer transistor 42. The latter has its emitter connected to the input 44 of a differential amplifier, generally indicated at 46, and its collector connected to current generating means, generally indicated at 48, for generating a current through transistor 42 so as to reduce the amount of bias current drawn by stage 18 from the input terminal 14A.

In particular the current generating means 48 includes transistors 50, 52, 54, 56 and 58. The base of transistor 50 is connected through junction 40 to the base of transistor 42 while the bases of transistors 52 and 54 are connected together. The emitter of transistor 54 is connected to the collector of transistor 42 while its collector is connected to the base and collector of diode-connected transistor 58. Transistor 52 has its collector coupled to the emitter of transistor 50 and its emitter connected to the collector and base of diode-connected transistor 56. The emitter of transistor 58 and the collector of transistor 50 are tied together to system ground while the emitter of transistor 52 and the collector and base of transistor 56 are connected to the collector of transistor 60. The latter has its base connected to the base and collector of transistor 62 which in turn has its emitter tied with the emitter of transistor 60 to the upper rail voltage terminal 64 (shown in FIG. 3B).

Differential amplifier 46 includes a pair of differential elements comprising transistors 66 and 68. Transistor 66 has its base connected to input 44, its collector connected through resistor 70 to the current mirror generally indicated at 72, and its emitter connected through capacitor 74 to system ground and through resistor 76 to junction 78. Transistor 68 has its base connected to system ground at junction 80, its collector connected to the current mirror 72, and its emitter connected to the collector and base of diode-connected transistor 82. Transistor 82 has its emitter connected through resistor 84 to junction 78. Current mirror 72 is provided by transistors 86 and 88 having their bases connected together and their emitters connected through the respective resistors 90 and 92 to the upper rail voltage terminal 64. The collector of transistor 86 is connected to resistor 86 and coupled to its base through capacitor 94. Transistor 88 has its collector connected directly to its base and to the collector of transistor 68. The output of differential amplifier 46 is provided at junction 96 between the collector of transistor 66 and resistor 70. Junction 96 is connected to the base of transistor 98 which in turn has its collector connected to the collector and base of transistor 62 and the base of transistor 60, and its emitter connected to transistor 100 shown in FIG. 3B. The emitter of transistor 98, junction 78 as well as junction 44 are suitably connected to the power source 32.

As described in our copending application Ser. No. 247,649 simultaneously filed herewith transistor 42 and current generating means 48 reduce the input bias current drawn from the input current at terminal 14A without affecting the gain bandwidth product of the stage. In particular, the bipolar transistor 42 provides a buffer between terminal 14A and the remaining portions of stage 18. Current generating means 48 generates current through bipolar transistor 42 so as to reduce the amount of bias current drawn by the remaining portion of the stage 18 without affecting the gain bandwidth product of the stage. Further, as described in our copending application Ser. No. 247,649 the use of bipolar transistor 42 makes the circuit topology easily adapted for IC implementation. The use of capacitor 94 and resistor 70 provide a zero (a zero being defined as the value of complex frequency at which the transfer function of the circuit becomes zero) in the transfer characteristics of the stage 18 so as to negate 90 phase shifts generated by the feedback loop of the stage provided by the VCA cell 20, therefore effecting the stability of the circuit. By connecting diode 82 to the emitter of transistor 68 and by matching the impedance of resistor 76 with the combined impedances of diode 82 and resistor 84, the noise contribution by the power supply 32 to the output at junction 96 is reduced. Finally, the addition of capacitor 74 introduces a zero in the transfer characteristics of the amplifier and the stage which will negate the pole (a pole being defined as the value of complex frequency at which the transfer function of the circuit becomes infinite) introduced by the base-to-emitter parasitic capacitance of transistor 66.

Referring to FIG. 3B transistor 100 has its collector connected to the upper rail voltage terminal 64 and its emitter connected through resistor 110 to junction 112. The latter forms one bias terminal of cell 20.

Cell 20 is described in greater detail in the copending application Ser. No. 247,829 simultaneously filed herewith by David E. Blackmer and assigned to the present assignee which application is a continuation-in-part of U.S. Application Ser. No. 131,220 filed Mar. 17, 1980. Junction 112 is connected through resistor 114 to the collector of secondary log transistor 116. The emitter of secondary log transistor 116 is connected to the emitter of primary log transistor 118. The collector of transistor 118 is connected to junction 120, which in turn is connected to the input terminal 14A of the module. Junction 120 is also connected to the collector of primary log transistor 122 which in turn has its emitter connected to the emitter of secondary log transistor 124. The collector of transistor 124 is coupled through resistor 126 to the junction 128. Junction 128 forms the second bias terminal of cell 20 so that resistor 114, secondary log transistor 116, primary log transistors 118 and 122, secondary log transistor 124 and resistor 126 form a circuit load for the bias generator 22. The junction 112 is also coupled through resistor 130 to the collector of secondary antilog transistor 132, which in turn has its emitter connected to the emitter of antilog transistor 134. The collector of transistor 134 is connected to the junction 136 which in turn is connected to the current output terminal 16A of the module. Current output terminal 16A thus provides the current output Iout of the module 10. Iout can easily be converted to the voltage Eout in a manner well known in the art. Junction 136 is also connected to the collector of primary antilog transistor 138, which in turn has its emitter connected to the emitter of secondary antilog transistor 140. Secondary transistor 140 has its collector coupled through resistor 142 to the junction 128. Resistor 130, secondary antilog transistor 132, primary antilog transistors 134 and 138, secondary antilog transistor 140 and resistor 142 form another circuit load between junctions 112 and 128 for bias generator 22. The base of primary log transistor 122 is tied with the base of primary antilog transistor 134 to the control signal terminal 144, which in turn receives the control signal from terminal of detector 12 described in reference to FIGS. 4A and 4B. The bases of transistors 118 and 138 can be respectively coupled to terminals 146 and 148 for receiving adjusting currents between positive and negative input signals to insure gain symmetry as will be more evident hereinafter. The bases of each secondary transistor 116 and 132 are each respectively cross-coupled to the other's collector and, similarly, the bases of each secondary transistor 124 and 146 are each cross-coupled to the other's collector.

As described in detail in the copending application Ser. No. 247,829 filed by David E. Blackmer, each primary log transistor 118 and 122 is respectively coupled to a secondary log transistor 116 and 124 to form two compound log devices, each disposed in a separate feedback path between the input and output of stage 18. Primary transistor 118 is a PNP transistor while primary transistor 122 is an NPN transistor. Each transistor 118 and 122 is connected to a secondary transistor of an opposite conductivity type. Accordingly, they function as "compound log transistors", each providing a log signal as a function of the logarithm of the corresponding one of two polarities of the input signal at terminal 14A.

As will be more evident hereinafter, the control signal at terminal 144 is a function of the logarithm of the instantaneous RMS value of the input signal at terminal 14A. The control signal is added to the log signals through the bases of primary transistors 122 and 134. Each primary antilog transistor 134 and 138 of opposite conductivity types is respectively coupled to a secondary antilog transistor 132 and 140 of an opposite conductivity type to form two compound antilog devices each for a different polarity of input signal. Transistors 132, 134, 138 and 140 function as "antilog transistors" for providing an antilog signal as a function of the antilogarithm of the sum of the corresponding log signal and control signal. The log transistors 116 and 118 and antilog transistors 132 and 134 thus form one log-antilog transmission path for one polarity of the input signal and log transistors 122 and 124 and antilog transistor 138 and 140 provide a second log-antilog transmission path for the other polarity of the input signal.

As described in copending application Ser. No. 247,829 filed simultaneously herewith by David E. Blackmer all of the NPN transistors are preferably matched for their Vbe/Ic transfer characteristics. Similarly, all of the PNP transistors are similarly matched to one another. The secondary transistors provide signal modification means disposed in the log-antilog path for reducing distortion due to the inherent base and emitter resistances of the log and antilog transistors. Correction is derived from a comparison between the input current and the output (antilog) current from each log-antilog path. Specifically, a correction signal is derived from the voltage differential generated between the collector voltages of the cross-coupled secondary transistors of each log-antilog path. The correction signal is provided to reduce the distortion at the output terminal 16A due to the inherent parasitic base and emitter resistances of the transistors of cell 20. Any mismatching of the parasitic and emitter resistances of each compound pair of transistors 116, 118; 122, 124; 132, 134; and 138, 140 can be easily compensated for by individually adjusting the respective resistors 114, 126, 130 and 142.

Bias generator 22 is suitably connected at junction 112 and junction 128 to provide a bias signal through the circuit load including transistors 116, 118, 122 and 124 and through the circuit load including transistors 132, 134, 140 and 142. Generator 22 is shown and described in greater detail in our copending application Ser. No. 247,648 filed simultaneously herewith and assigned to the present assignee. Generator 22 is designed to provide a bias voltage between junctions 112 and 128 of the cell 20, programmable by an input current from the power source 32 to the generator 22 such that the resulting biasing current through the log transistors and the resulting biasing current through the antilog transistors will be equal or proportional to, and thus track, the input current independent of temperature.

More specifically, generator 22 includes a first resistor 150 having one end connected to the junction 112 and its other end connected to junction 154. Resistor 152 is coupled with resistor 150 between junction 154 and the junction 128. Junction 112 is also connected to the collector of NPN transistor 156 which in turn has its emitter connected to the emitter of PNP transistor 158. NPN transistor 156 is matched for its Vbe/Ic transfer characteristics to NPN transistors 116, 122, 132 and 138 and PNP transistor 158 is similarly matched to PNP transistors 118, 124, 134 and 140 of cell 20. The base of transistor 158 is connected to junction 154, while its collector is connected to the base of transistor 160. Transistor 160 has its emitter connected to the base of transistor 162. The collectors of transistors 160 and 162 are connected together to the base and collector of transistor 156 and to the junction 112. The emitter of transistor 162 is connected to the junction 128. The base of transistor 158 is connected to the anode of diode 164 which in turn has its cathode connected to the base of transistor 160. The base of transistor 160 is connected to a source of constant current which is provided by power supply 32.

As described in our copending application Ser. No. 247,648 simultaneously filed herewith the current from source 32 to the junction of the collector of transistor 158 and the base of transistor 160 is the programming input current to the generator. The collector current of transistor 158 is equal (neglecting the base current of the transistor 158) to and is thus dependent on the current through the base-to-emitter junctions of both transistors 156 and 158. The transistors 160 and 162 form a plus one follower having voltage gain of one and very high current gain.

In operation, the current from power source 32 to the base of transistor 60 remains constant and establishes the level of the bias voltage applied across junctions 112 and 128. The amount of current flowing through the emitters of transistors 156 and 158 establish the temperature dependent voltage across resistor 150. The collector current of transistor 158 is approximately equal to the emitter current of transistor 158 and is equal to the input current to the generator 22 so that the error current input to the base of transistor 160 is approximately zero. If a change would occur in the emitter current in transistors 156 and 158 compensation would be provided through transistors 160 and 162 to adjust for the error. Specifically, if the emitter currents through transistors 156 and 158 increased, the voltage across resistor 150 would increase. The collector current of transistor 158 would in turn increase. Since the input current from power source 32 remains constant, an error current equal to the difference between the collector current of transistor 158 and the input current would result, and the voltage level on the base of transistor 160 would decrease with respect to the voltage level at junction 112. This in turn would reduce the current through resistors 152 and 150, reducing the voltage across the base-emitter junctions of transistors 156 and 158, which in turn would result in a reduction in the emitter currents in transistor 156 and 158 until the collector current of transistor 158 equalized with the input current from source 32 and the error current in the base of transistor 160 equaled zero.

Conversely, if the emitter currents of transistors 156 and 158 were to decrease and thus the voltage across resistor 150 decreased and the collector current of transistor 150 decreased, an error current equal to the difference between the collectors current of transistor 158 and the input current would be applied to the base of transistor 160. This would cause an increase in the voltage level of the base with respect to the junction 112. This in turn would increase the current through resistors 152 and 150 and the voltage across the base-emitter junctions of transistors 156 and 158. This in turn would increase the emitter currents in transistors 156 and 158 until the collector current of transistor 158 equalized with the input current from sources 32 and the error current in the base of transistor 160 equaled zero.

A temperature-dependent voltage is therefore generated across resistor 150 which is the sum of the voltage drops across the base to emitter junctions of transistors 156 and 158. The temperature voltage drop across each circuit load between junctions 112 and 128 of the cell 20 varies as a function of the sum of the voltage across the base-to-emitter junctions of two NPN and two PNP transistors matched respectively to the NPN and PNP transistors 156 and 158.

Therefore, in order to match the voltage current temperature characteristics of the generator 22 with the voltage current temperature characteristics of each circuit load provided between the junctions 112 and 128, it is necessary to multiply the voltage generated across resistor 150 by the scalar two. This is accomplished by setting the resistance value of resistor 152 equal to the resistance value of resistor 150 so that the current generated through resistor 150, responsive to the voltage generated by transistors 156 and 158 there across, will be transmitted through resistor 152 (neglecting the base current of transistor 158) to generate a like voltage. The voltages across resistors 150 and 152 add together to provide the bias signal across each circuit load between junctions 112 and 128 of cell 20. It will be appreciated that where the gain control module is subjected to a change in ambient temperature, a change in the base-emitter voltage drops of transistors 156 and 158 will be matched by a like change in the base-emitter voltage drops of each pair of PNP and NPN transistor in each circuit load.

Referring to FIG. 3A the current source 32 includes transistor 170 having its collector connected to the base of transistor 160 of bias generator 22. Transistor 170 has its emitter connected through resistor 172 to junction 174. Transistor 170 is cascaded with transistors 176 and 178 by having their bases connected together. Transistor 176 has its collector connected to the emitter of transistor 177. Transistor 177 has its base connected to system ground and its collector connected to the emitter of transistor 98. Transistor 178 has its collector connected to junction 78 of stage 18. The emitters of transistors 176 and 178 are directly connected to junction 174. The bases of transistors 170, 176 and 178 are coupled through resistor 180 to the base of transistor 182 and the collector of transistor 184. The collector of transistor 182 is directly connected to junction 44 of stage 18. The emitter of transistor 182 is connected to the emitter of transistor 170. The bases of transistors 170, 176 and 178 are coupled through resistor 186 to the base of transistor 184. The base of transistor 184 is suitably connected two diode drops below system ground by connecting the base through diode-connected transistors 188 and 190 to system ground. Junction 174 is connected to the base of transistor 192 and through resistor 194 to terminal 196. The emitter of transistor 192 is also connected to terminal 196 while its collector is connected through resistor 198 to junction 128 of cell 20. Terminal 196 can be suitably coupled through external resistor 200 to a voltage source 202. The nature of power supply 32 is such that a constant current is always provided on the collector of transistor 170 so that bias generator 22 functions in accordance with the teachings of our copending application Ser. No. 247,648 filed simultaneously herewith.

Referring to FIGS. 4A and 4B the detector 12 is shown in detail. In particular in addition to the rectifier 24 (FIGS. 4A and 4B), log section 26 (FIGS. 4A and 4B), log filter 28 (FIG. 4B) and buffer amplifier 30 (FIG. 4B), the detector also includes current sources 300 and 302 (each shown partially in FIGS. 4A and 4B) and voltage source 304 (FIG. 4A).

FIG. 4A the voltage input terminal 14 is connected through capacitor 310 to the resistor 312, which in turn is connected to the current input terminal 14B (receiving the current input signal Iin). Current input terminal 14B is connected to the input of rectifier 24.

Rectifier 24 comprises the operational amplifier stage 320 and the bias generator 324. Rectifier 24 is constructed in accordance with the teachings described in copending application Ser. No. 247,811 filed by David R. Welland simultaneously herewith and assigned to the present assignee.

Specifically current input terminal 14B is connected to the input junction of terminal 330 of the stage 320. Junction 330 is connected to the base of transistor 332, the latter having its collector connected to system ground and its emitter connected to the collector of transistor 334. Junction 330 is also connected to the base of transistor 336, the latter having its emitter connected to junction 338 and its collector connected to the emitter of transistor 340. Transistor 340 has its base connected to the base of transistor 334 and its collector connected to the emitter of transistor 342. The base of transistor 342 is connected with the emitter of transistor 334 to the voltage source 304, while the collector of transistor 342 is connected to the collector of transistor 344 and the base of transistor 346. Transistor 344 has its base connected to the base and collector of transistors 348 and its emitter tied with the emitter of transistor 348 to the positive voltage rail indicated at 350. The collector of transistor 348 is connected through capacitor 352 to the base of transistor 346. The collector of transistor 348 is also connected to the collector of transistor 354 which in turn has its base connected at 355 to the system ground and its emitter connected to junction 338. The transistor 346 has its collector connected to the positive voltage rail 350 and its emitter connected to the base of transistor 356 and directly to the current sources 302. Transistor 356 has its collector connected to the positive voltage rail 350 and its emitter connected to an anode of a first diode 358 which in turn has its cathode connected to the anode of a second diode 360. Diode 360 has its cathode forming the output of stage 320 connected to the rectifier cell 322 such that the cell is connected in a feedback path of the amplifier stage 320 as taught in the copending application Ser. No. 247,811 filed by David R. Welland simultaneously herewith.

Cell 322 comprises three transistors 380, 382 and 384. Transistor 380 is an NPN transistor, diode connected by having its base connected through resistor 386 to its collector to form input 388 of cell 322. Input 388 is connected to the input of amplifier stage 320 at junction 330. Transistor 382 is also an NPN transistor having its collector connected to the cathode of diode 398, which in turn has its anode connected to the output 400 of the cell 388. Transistor 382 has its emitter tied to the emitter of transistor 380 and its base connected through resistor 390 to system ground. The base of transistor 382 is also connected to the terminal 392, which in turn is adapted to be connected through a resistor 394 to a potentiometer 396. The latter provides a variable current to the base of transistor 382 to insure gain symmetry between the current paths provided by transistor 380 and 382 on the one hand and transistor 384 on the other hand as described in the copending application Ser. No. 247,811. Finally, transistor 384, an NPN transistor, has its emitter connected to the input 388 of the cell, its base connected to the cathode of diode 360 of stage 320 and its collector connected to the output 400 of the cell. As described in the copending application Ser. No. 247,811 the loop transmission of the stage 320 through each of the path involving transistor 380 and the path involving transistor 384 will limit at unity gain thereby eliminating stability problems associated with unlimited loop transmission in the rectification of positive input signals by the rectifier disclosed in U.S. Pat. No. 4,097,767.

The bias generator 324 provides a bias voltage between the base of transistor 384 and the tied emitters of transistors 380 and 382 as described in detail in the copending application Ser. No. 247,811. Generator 324 comprises signal generating means for generating the bias voltage between the points 410 and 412. The bias signal is applied across a first impedance load comprising first resistive means in the form of resistor 440. Resistor 440 is coupled to second resistive means, resistor 442, which forms part of a second impedance load connected between the base of transistor 384 and the tied emitters of transistors 380 and 382. As will be more apparent hereinafter by making resistors 440 and 442 of equal resistance the bias voltage is also applied between the base of transistor 384 and the tied emitters of transistors 380 and 382.

More particularly, the signal generating means includes a first current source including transistors 414 and 416 for providing the reference current IB, a second current source including transistors 418 for providing a reference current IA, a pair of reference transistors 420 and 422 and a second pair of reference transistors 424 and 426. Transistor 414 has its emitter tied with the emitter of transistor 416 and the emitter of transistor 418 to system ground. The collector and base of transistor 414 are connected to the current sources 302 and to the bases of transistors 416 and 418. The collector of transistor 416 is connected to the base of transistor 434, which in turn has its collector connected to the output of the log section 26 and its emitter connected to the base and collector of diode-connected transistor 436. The latter has its emitter connected to the point 412. The collector of transistor 416 is also connected to the collector of transistor 420, which in turn has its base connected to the point 410 and its emitter connected to the collector and base of diode connected transistor 422. The emitter of transistor 422 is connected to the negative voltage rail 450 and the resistor 428. The collector of transistor 418 is connected to the base and collector of diode-connected transistor 432 which in turn has its collector connected to point 410. Point 410 is connected to the base and collector of diode-connected transistor 424, the latter having its emitter connected to the base and collector of diode-connected transistor 426. The emitter of transistor 426 is connected to the common junction of resistors 428 and resistor 430. Resistor 430 is in turn connected to point 412.

The base of transistor 432 is connected to the base of transistor 438, which in turn has its emitter connected through resistor 440 to the point 412. The collector of transistor 438 is connected to the base and collector of diode-connected transistor 444, the latter having its emitter connected to resistor 442 which in turn is connected to the base of transistor 384 of the cell 322. The collector and base of transistor 444 are connected to the base of transistor 446 which in turn has its emitter connected to the tied emitters of transistors 380 and 382 and its collector connected to the negative voltage rail 450 of the current sources 302. As described in the copending application Ser. No. 247,811 transistors 420, 422, 424 and 426 are all NPN transistors matched for their Vbe/Ic characteristics with those of the NPN transistors 380, 382 and 384 of the rectifier cell 322. Similarly, transistors 432 and 438 are of like conductivity type and are matched for their Vbe/Ic characteristics and transistors 444 and 446 are of the same conductivity type and matched for their Vbe/Ic characteristics.

The rectifier 24 operates in a manner consistent with the teachings of copending application Ser. No. 247,811 simultaneously filed herewith by David R. Welland. In general, when the input current at junction 14A is negative, the output signal at the cathode of diode 360 of stage 320 is positive. This provides negative feedback through the base-emitter junction of transistor 384 of the cell 322. This, in turn results in current flowing from the output at terminal 400 of the rectifier, through the collector-base junction of transistor 384 to the input terminal 330 of stage 320. Transistors 380 and 382 are substantially nonconductive since their base-emitter junctions are reversed biased.

For positive input currents at terminal 14B the output of stage 320 is negative, the base-emitter junction of transistor 380 is forward biased and current flows through the junction. A substantially equal mirrored current flows through the collector of transistor 382 to therefore provide an inverted signal at the output 400 for this polarity of the input signal.

The base of transistor 384 and the tied emitters of transistors 380 and 382 are biased by the bias generator 324 so as to reduce the slewing requirements of amplifier 320 for a given level of performance.

As taught in the copending application Ser. No. 247,811 the bias voltage generated between points 410 and 412 is equal to the voltage drop across the base-emitter of transistor 424 plus the voltage drop across the base-emitter of transistor 426 minus the voltage drop across resistor 430. In general, if the reference current IB through transistors 420 and 422 is equal to n times the reference current IA through transistors 424 and 426, the voltage across resistor 428 will be that which when subtracted from the voltage across a reference diode string (such as provided by transistors 420 and 422) will cause the current through a matched string to be reduced by a factor of n. The voltage across resistor 430 then will be that which when subtracted from the voltage across the reference diode string of transistors 424 and 426 will cause the current through the matched string to be reduced by a factor of n raised to the kth power (wherein k is the ratio of the resistance of resistor 430 to that of resistor 428). Since the voltage across point 410 and 412 is the voltage drop across the diode string consisting of diode-connected transistors 424 and 426 minus the voltage drop across resistor 430, and since transistors 424 and 426 are matched to transistors 420 and 422, as taught in the copending application in U.S. Ser. No. 137,427 filed on Apr. 4, 1980, and the copending application Ser. No. 247,811 the circulating current provided to transistors 384 and 380 (the current circulating through base and emitter of transistor 384 and the collector and emitter of transistor 380) will be a factor of n raised to the kth power less than the reference current through transistors 424 and 426. The bias voltage therefore can vary with temperature in such a manner such that the circulating current will not change with such temperature variations while the setting of current IA, the ratio of IB to IA, as well as the ratio of resistors 428 and 430 will set the maximum circulating current generated in the collector and emitter of transistor 380 and the base-emitter junction of transistor 384 and the maximum error at output 400 due to this circulating current.

The bias voltage across points 410 and 412 thus equals the desired bias voltage. The voltage generated across these points is applied across the first impedance load comprising the base emitter junctions of transistors 432 and 438 and the resistor 440. The base-emitter junction of transistor 438 is forward biased so as to be conductive. The bias voltage provided across resistor 440 therefore generates a current through the resistor 440 and the transistor 438 which in turn forces the current through resistor 442 and the diode-connected transistor 444. The current through the emitter of transistor 446 is set so as to be the same as that through transistor 432 (IA) so that by matching transistor 444 to transistor 446 and matching transistor 432 to transistor 438, any mismatching between the voltage drops across each of transistors 432 and 438 due to a mismatching of currents flowing through the collectors of these transistors will be offset by a similar mismatch in voltage drops between transistors 444 and 446 due to a duplication in the current mismatching. The current through resistor 442 results in the bias voltage being generated across a second impedance load comprising the resistor 442 and the base-emitter junctions of transistors 444 and 446. Thus, the voltage across resistor 442 and transistors 444 and 446 equals the required bias voltage which can vary with temperature without effecting the circulating current in transistors 380 and 384.

By providing the proper bias voltage to the base of the transistor 384 and the common emitters of transistors 380 and 382, the circulating error current transmitted at the output 400 due to the circulating current flowing in the base emitter path of transistor 384 and the collector emitter path of transistor 380 is temperature independent and the maximum acceptable level the circulating error current at output 400 is determined by the ratio of the resistance values of resistors 428 and 430, the bias current IA conducted through transistors 424 and 426 of the generator 324, and the ratio of currents IB to IA.

Therefore, the current output signal of rectifier 24 at output 400 is a full wave rectification of an input current at terminal 14B. The output 400 is connected to the logging section 26.

Logging section 26 (shown in FIGS. 4A and 4B) includes an operational amplifier stage 451 and logging elements 452. Stage 451 includes an input 454 connected to the output 400 of the cell 322 of rectifier 24. Input 454 forms the connection between the bases of transistors 456 and 458. The collector of transistor 456 and the emitter of transistor 458 are connected to system ground. The emitter of transistor 456 is connected to the collector of transistor 458 which in turn has its emitter connected to the voltage source 304, the current sources 300. The emitter of transistor 458 is also connected to the base and collector of transistor 460 and the anode of diode 461. The collector of transistor 460 is connected to current sources 300 and the cathode of diode 461. The emitter of transistor 460 is connected to the collector of transistor 462 which in turn has its base connected to the base of transistor 458 and its emitter connected through resistor 464 (shown in FIG. 4B) to the collector of transistor 458. The collector of transistor 460 is connected through capacitor 468 to system ground and to the base of transistor 466. The collector of transistor 466 is connected to the current sources 300. The emitter of transistor 466 is connected to the base of transistor 470 which in turn has its collector connected to the sources 300 and its emitter connected to the output of logging elements 452.

Logging elements 452 are formed by connecting two diode-connected transistors 478 and 480 in a feedback loop between the output and input of the stage 451, the output and inputs being respectively formed by the emitter of transistor 470 and the input 454. In operation the rectified current signal at input 454 will always flow into the rectifier 24 so that the transistors 480 and 482 of the logging elements 452 are always forward biased. The nature of stage 451 and logging elements 452 convert the rectified signal into a signal which is a function of the square of its input signal or more accurately a function of two multiplied by the logarithm of the input signal. Thus, the output at terminal 476 of stage 451 is a representation of the logarithm of the square of the current input signal at current input terminal 14B.

The output at the emitter of transistor 470 of the stage 451 is connected to the input of logging filter 28 (FIG. 4B). Logging filters for use in the log domain are taught in the copending application Ser. No. 97,901 filed by Robert W. Adams on Nov. 28, 1979, now abandoned and assigned to the present assignee. In particular filter 28 includes at least one diode connected transistor 490. The output of the log section 26 is connected to the base and collector of transistor 490 while the emitter of the transistor forms the output of the filter. The output of the filter is suitably biased by the current sources 302 and is connected through external capacitor 492 to the external resistor 494 which in turn is connected to system ground.

As taught in the copending U.S. application Ser. No. 97,901, now abandoned, since the output of the log section 26 is a log signal, filter 28 functions as a low pass filter and the output of the filter is primarily a DC signal. This DC signal is provided to the input of buffer amplifier 30.

Amplifier 30 comprises a transistor 500 having its base connected to the output of filter 28. The emitter of transistor 500 is connected to the base and collector of diode-connected transistor 502 which in turn has its emitter connected to the base and collector of diode-connected transistor 504. Transistor 504 has its emitter connected to junction 506, which in turn is connected to sources 302. The collector of transistor 500 is connected to the collector and base of transistor 508, the latter having its emitter connected through resistor 510 to the positive voltage rail 350. The base and collector of transistor 508 is also connected to the base of transistor 512, which in turn has its emitter connected through resistor 514 to current sources 300. The collector of transistor 512 is connected to the base of transistor 516 and the collector of transistor 518. Transistor 516 has its collector connected to the positive rail 350 and its emitter connected to the sources 302 and the base of transistor 520. Transistor 520 has its collector connected to the collector and base of transistor 522, which in turn has its collector and base connected to current sources 300 and its emitter connected to rail 350. Transistor 520 also has its emitter connected to the base of transistor 518 which in turn has its emitter connected to the junction 506. The base of transistor 518 and the emitter of transistor 520 collectively form the output 524 of the detector 12. Output terminal 524 is suitably biased by current sources 302 and is connected to terminal 144 of the cell 20 of the control module shown in FIG. 3.

Current sources 300 and 302 and voltage source 304 respectively provide the necessary bias currents and voltage to the rectifier 24, logging section 26, logging filter 28 and buffer amplifier 30. In particular referring to FIGS. 4A and 4B, sources 300 includes four PNP transistors 600, 602, 604 and 606 all having their emitter connected to the positive voltage rail 350. The collector of transistor 600 is connected to the tied emitters of transistors 380 and 382 of rectifier cell 322 of the rectifier 24. The base of transistor 600 is tied to the bases of transistors 602 and 604. Transistor 602 has its collector connected to the collector of transistor 460 and base of transistor 466 of the log section 26. Transistor 604 has its collector and base connected to the current sources 302. Finally, transistor 606 has its base connected to the base and collector of transistor 522 of amplifier 30 and its collector connected to the base of transistor 460 of the log section 26.

Referring to FIG. 4A, current sources 302 includes diode-connected transistor 620 having its collector and base forming the output terminal 622. Terminal 622 is connected through external resistor 624 to system ground. Diode-connected transistor 620 has its emitter connected to the base and collector of diode-connected transistor 622, which in turn has its emitter connected to the collector of transistor 624. The latter has its emitter connected to the negative voltage rail 450. The base and collector of transistor 622 (two diode drops above the negative voltage rail 450) is connected to the base of transistor 626 and the base of transistor 628. The emitter of transistor 626 is connected to the base and collector of transistor 624 (one diode drop above the negative voltage rail 450). Similarly, the emitter of transistor 628 is connected to the collector and base of transistor 624. The collector and base of transistor 624 are in turn connected to the emitters of transistors 630, 632, 634, 636, 638 and 640 and to the base of transistor 642, the latter having its emitter connected to the negative voltage rail 450. The common base connection of transistors 626 and 628 is connected to the bases of transistors 630, 632, 634, 636, 638 and 640. The transistor 630 has its collector connected to the emitter of transistor 644, which in turn has its base connected to the base of transistor 646. Transistor 646 has its emitter connected to the collector of transistor 632. The collector of transistor 626 is connected to the junction 338 of stage 320, while the collector of transistor 628 is connected to the base collector of transistor 414 of bias generator 324. The collector of transistor 644 is connected to the emitter of transistor 346 and the base of transistor 356 of amplifier stage 320. The collector of transistor 646 is connected to the emitter of transistor 466 and the base of transistor 470 of the log section 26. Transistor 634 has its collector connected to the common collector and base of transistor 604 and sources 300. Transistor 636 has its collector connected to the output of filter 28 while the collector of transistor 638 has its collector connected to junction 506 of buffer amplifier 30. Transistor 640 has its collector connected to the base of transistor 520 and the emitter of transistor 516 of the buffer amplifier 30, while the collector of transistor 642 has its collector connected to the output terminal 524 of the detector.

Voltage source 304 includes three diode-connected transistors 700, 702 and 704. Transistor 700 has its collector and base connected three diode drops above system ground to the emitter of transistor 458 and the base of transistor 460 of log section 26. The emitter of transistor 700 is connected to the collector and base of transistor 702 so as to form a source two diode drops above system ground. The collector and base of transistor 702 is connected to the emitter of transistor 334 and the base of transistor 342 of stage 320. Finally, transistor 702 has its emitter connected through the diode-connected transistor 704 to ground.

In operation the system is properly biased so that the necessary biasing currents are provided by source 32 of the gain control module and current sources 300 and 302 of the detector. When an input voltage signal is applied to input terminal 14 it is converted to a current which is applied to terminals 14A and 14B. Thus, current signal at terminals 14A and 14B is presented simultaneously to the gain control module 10 and the detector 12.

The detector receives the signal at junction 330 whereupon the signal is rectified by rectifier 24. Specifically, a negative signal at input 330 results in a positive output of amplifier stage 320. This results in transistor 384 being forward biased and current flowing from the output 400 of the cell 322 through the collector-emitter path of transistor 384 of rectifying cell 322. Thus, a current flows from input terminal 454 of logging section 26. The transistors 380 and 382 of cell 322 remain reversed biased and thus nonconductive. Positive current signals at input 330 result in a negative output of amplifier stage 320. This results in transistor 380 being forward biased and transistor 384 being reversed biased. Transistor 380 thus conducts a current which results in a mirrored current through the collector-emitter path of transistor 382. The emitter currents of transistors 380 and 382 are sunk through the bias generator to the negative voltage rail 450. Thus, as in the case of negative input signals, current flows from input terminal 454 of logging section 26 to the cell 322. Full wave rectification is therefore provided. The current signal provided at input terminal 454 of logging section 26 is converted to a log signal and multiplied by the factor two through the feedback path of diode-connected transistors 480 and 482. This results in an output signal of the log section which is a representation of the logarithm of the square of instantaneous input current at terminal 14B. The signal at the output of log section 26 is transmitted through low pass filter 28; thence through amplifier 30 to provide the control signal at terminal 524. Terminal 524 is connected to terminal 144 so that the control signal can be added to the log signal in the VCA cell 20 of the module.

The input signals at terminals 14A and 14B are also transmitted through module 10 where a gain is impressed on the current signal as a function of the control signal. For negative input signals at terminal 14A, feedback occurs through log transistors 116 and 118 of VCA cell 20. The voltage signal across primary and secondary log transistors 116 and 118 are a function of the logarithm of the input signal since the transistor lies in the feedback path of stage 18. The control signal at terminal 144 is arithmetically added to the log signal before the algebraic sum of the two signals is converted by antilog transistors 132 and 134 to an antilog signal as a function of the antilogarithm of the sum of the two signals. For positive input signals at terminals 14A, feedback occurs through log transistors 122 and 124 of VCA cell 20. The log signal provided by log transistors 122 and 124 is again a function of the logarithm of the input signal since the transistor also lies in the feedback path of stage 18. The control signal at terminal 144 is added to the log signal before the algebraic sum of the two signals is converted by antilog transistors 138 and 140 to an antilog signal which is a function of the antilogarithm of the sum of the two signals.

The use of stage 18 and in particular transistor 42 and current generating means 48 results in a smaller bias current drawn by the stage from terminal 14A without affecting the gain bandwidth product of the stage.

Further, the use of capacitor 94 and resistor 70 provide a zero in the transfer characteristics of the amplifier 18 to negate 90.sup.• phase shifts produced in the feedback paths through the VCA cell 20. By connecting diode 82 to the emitter of transistor 68 and by matching the impedance of resistor 76 with the combined impedance of diode 82 and resistor 84, any noise contribution by source 32 is reduced. Finally, the addition of capacitor 84 introduces a zero in the transfer characteristics of the stage which will negate the pole introduced by the base-to-emitter parasitic capacitance of transistor 66.

The use of bias generator 22 provides a bias voltage between the junctions 112 and 128 of the VCA cell 20. The bias voltage is determined by the input current from source 32 and generates a biasing current through the cell which is independent of temperature.

The use of the secondary transistors 116, 124, 132 and 140 and resistors 114, 126, 130 and 142 reduce distortion due to parasitic base and emitter resistances of the log and antilog transistors 118, 122, 134 and 138 of cell 20.

The use of the rectifying cell 322 of the detector 12 and in particular providing maximum limit of unity loop gain in both feedback paths around the amplifier stage 320, improves the performance of the detector. Finally, the bias generator 324 provides the bulk of the generator away from the base of transistor 384 and the common emitters of transistors 30 while at the same time providing the desired bias voltage therebetween.

It will be appreciated that the foregoing provides an improved compander system which can be used either as a compressor or an expander of the signal applied to the input terminals 14A and 14B. The system is easily manufacturable as an IC circuit providing reduced costs of manufacture. When manufacturing the module 10 and detector 12 as separate integrated circuit chips both will dissipate a substantially like amount of power. More particularly, the output of detector 12 is a function of the RMS value of the input signal and operating temperature. The gain of the detector is a function of the control signal and temperature. The present design therefore is such that variations of the RMS output with temperature are matched to changes in VCA gain with temperature such that the compression or expansion factor will be independent of temperature so long as the module and detector are operated at the same temperature.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not in a limiting sense. 

What is claimed is:
 1. A signal conditioning system comprising, in combination:(A) a gain control module for varying a gain impressed upon an input signal as a function of a control signal, said module comprising:(1) a first operational amplifier stage having an input terminal coupled to receive said input signal and an output terminal; (2) first log conversion means disposed in each of two feedback paths coupling said input terminal of said amplifier stage with said output terminal for providing a log signal as a function of the logarithm of a corresponding one of two representation of said input signal; (3) antilog conversion means coupled to each log conversion means for providing an antilog signal as a function of the antilogarithm of the sum of the corresponding log signal and said control signal; and (4) first bias generating means for generating a bias current through said log conversion means of both said feedback paths and through said antilog conversion means; and (B) control signal generating means for detecting said input signal and for generating said control signal responsively thereto, said control signal generating means comprising(1) operational rectifier means for providing a rectified signal which is a substantial rectification of said input signal, said rectifier means having an input terminal adapted to receive said input signal and an output terminal for providing said rectified signal, said rectifier means comprising(a) a second operational amplifier stage having an input terminal coupled to the input terminal of said rectifier means and an output terminal; (b) first current conduction means responsive to the output of said second operational amplifier stage and defining a transmission path between the input and output terminals of said rectifier means for substantially conducting one representation of said input signal between the input and output terminals of said rectifier means; (c) second current conduction means responsive to the output of said second operational amplifier stage and defining a second transmission path between the input and output terminals of said second operational amplifier stage for substantially conducting the other representation of said input signal between the input and output terminals of said second operational amplifier stage; (d) signal generating means responsive to said input signal when substantially conducted through said second transmission path for generating an inverted signal of said other representation of said input signal; (2) second log conversion means coupled to the output of said operational rectifier means for providing a second log signal as a function of the logarithm of said rectified signal; and (3) means including a low pass filter coupled to the output of said second log conversion means for providing said control signal substantially as a function of the DC value of said second log signal.
 2. A system according to claim 1, wherein said first and second current conduction means each have a maximum loop transmission at unity gain.
 3. A system according to claim 1, wherein said second bias generating means of said operational rectifier means includes a first impedance load comprising a first resistive means; a second impedance load coupled to said first and second current conveying means and including second resistive means coupled to said first resistive means; and means for generating a voltage across said first impedance load so that a current is generated in said first resistive means, a current is generated in said second resistive means in response to said current in said first resistive means, and a biasing voltage is generated across said second impedance load, wherein said biasing voltage varies with temperature such that bias current generated in said first and second current conduction means is independent of temperature.
 4. A system according to claim 1, wherein said low pass filter includes at least one diode means connected between the input and output of said filter.
 5. A system according to claim 1, wherein said first bias generating means includes means for maintaining said bias current substantially independent of temperature.
 6. A system according to claim 5, wherein said first biasing generating means includes reference means for providing a first voltage as a function of temperature, and scalar means for multiplying said first voltage to provide a bias voltage across said log conversion means of both said feedback paths and across said antilog conversion means so as to generate said bais current.
 7. A system according to claim 6, wherein said reference means includes the base-emitter junction of at least one transistor and said scalar means includes the ratio of two resistors.
 8. A system according to claim 1, wherein said control signal generating means further includes second bias generating means coupled to the output of said second operational amplifier stage for biasing said first and second current condition means and said signal generating means so as to reduce the slew rate requirements for a given level of performance of said second operational amplifier stage.
 9. A system according to claim 8, wherein said second bias generating means of said operational rectifier means includes means for generating a circulating current through said first and second rectification means such that the current error generated at the output terminal of said operational rectifier means in response to said circulating current is temperature independent.
 10. A system according to claim 9, wherein said means for generating said circulating current includes said means for preselecting the maximum acceptable level of said current error, said means for preselecting including the ratio of a pair of resistors.
 11. A system according to claim 1, wherein said first operational amplifier stage includes means for reducing the input bias current drawn from said input signal without appreciably affecting the gain bandwidth product of said stage.
 12. A system according to claim 11, wherein said means for reducing said input bias current includes bipolar transistor means disposed at the input terminal of said first operational amplifier stage for buffering said input terminal from the remaining portion of said stage, and second signal generating means for generating a current through said bipolar transistor means so as to reduce the amount of bias current drawn by said remaining portion of said stage.
 13. A system according to claim 1, wherein each said first log conversion means and the antilog conversion means coupled thereto define a log-antilog transmission path, said gain control module further comprising signal modification means disposed in each log-antilog path for modifying said input signal and said antilog signal in accordance with a correction signal so as to reduce distortion in said antilog signal.
 14. A system according to claim 13, wherein said first log conversion and said antilog conversion means of each said log-antilog transmission path respectively include transistors of the same conductivity type, with the transistors of one path each being of an opposite conductivity type from those of the other path.
 15. A system according to claim 14, further including means for adding said control signal to said log signal when said input signal is of one representation, and to said antilog signal when said input signal is of the other of said representations.
 16. A system according to claim 15, wherein said means for adding said control signal includes coupling means for adding the control signal to the base of the transistor of the first log conversion means of one log-antilog transmission path and the base of the transistor of the antilog conversion means of the other log-antilog conversion means.
 17. A system according to claim 14, wherein said signal modification means includes a transistor coupled to each transistor of each of said first log conversion means and each of said antilog conversion means of each log-antilog transmission path so as to form a transistor pair, the transistors of each pair being of opposite conductivity types.
 18. A system according to claim 17, wherein all of said transistors of each conductivity type are matched for their Vbe/Ic transfer characteristics.
 19. A system according to claim 18, wherein the emitters of each transistor pair are coupled together, and said signal modification means includes means for generating a correction signal through each of said transistor pairs for correcting for the parasitic base and emitter resistances of said transistor pair.
 20. A system according to claim 19, wherein said means for generating said correction signal includes means for detecting the difference between the input signal and the corresponding antilog signal of each path.
 21. A system according to claim 20, wherein said means for detecting said difference includes resistance means connected to the collector of each transistor of each said signal modification means and means for measuring the voltage differential between the collectors of the transistors of the signal modification means in each said path.
 22. A system according to claim 21, wherein said means for measuring includes means for cross-coupling the base of each transistor of the signal modification means in each log-antilog transmission path to the collector of the other transistor of the signal modification means in the same log-antilog path.
 23. A system according to claim 22, wherein each of said resistance means is adjustable to correct for any mismatching in the parasitic base and emitter resistances of said transistor pair.
 24. A system according to claim 1, wherein said first current conduction means of said operational rectifier means includes a transistor having its collector and emitter connected to conduct current from the output terminal of said operational rectifier means to the input terminal of said operational rectifier means, said second current conduction means of said operational rectifier means includes a second transistor having its base-emitter junction connected to conduct current from the input terminal of said operational rectifier means to the output terminal of said second operational amplifier stage, and said signal generating means of said operational rectifier means includes a third transistor having its collector and emitter connected to conduct current, in response to current flowing through the base-emitter junction of said second transistor, from the output terminal of said operational rectifier to the output terminal of said first operational amplifier stage.
 25. A system according to claim 24, further including means for providing gain symmetry between said second and third transistors on the one hand and said first transistor on the other hand.
 26. A system according to claim 1, wherein said second log signal is a function of the logarithm of the square of the instantaneous value of said input signal.
 27. A system according to claim 26, wherein said second log conversion means comprises a third operational amplifier stage having an input terminal for receiving said rectified signal and an output terminal for providing said second log signal, and a pair of diode means connected between the input and output terminals of said third operational amplifier stage. 